Various techniques of hiding one image within the other, visible image have been developed. U.S. Pat. No. 5,710,636 discloses a technique based on distributing pixels of an image to be hidden in between the pixels of a visible image, such that the hidden image can be identified by significant enlargement of the visible image. According to the techniques disclosed in U.S. Pat. Nos. 4,627,819 and 4,889,367, physical filtering means are used to separate between the hidden and visible images. This is implemented by using different recording materials to record the hidden image within the visible image, and using different kinds of incident radiation to read these images.
Another techniques of the kind specified utilize halftone coding methods. According to the technique disclosed in U.S. Pat. Nos. 5,329,108 and 5,416,312, the halftone dots (bits) of an image to be hidden are encoded and located within a predetermined area of a visible image. This predetermined area is selected as a background area of the visible image. U.S. Pat. No. 5,315,098 discloses a technique for encoding digital data in the angular orientation of circularly asymmetric halftone dot patterns that are written into the halftone cells of digital halftone images. According to this technique, each of the cells of digital data representative of a visible image is modified to encode the corresponding halftone dot of an image, which is to be hidden.
Halftone coding is a common method of representing continuous-tone images by binary values. As known [O. Bryngdahl et al., “Digital halftoning: synthesis of binary images,” in Progress In Optics, E. Wolf, ed. North-Holland, Amsterdam, 1994, Vol. 33, pp. 389-463], according to the halftone binarization technique, various tone levels are translated to the area of binary dots. This method, known as binarization by a carrier [D. Kermisch and P. G. Roetling, “Fourier spectrum of halftone images,” J. Opt Soc. Am. A 65, 716-723 (1975)], is related to the pulse-width modulation in communication theory.